The error - TypeError: 'str' object is not callable - is very clear in itself.

By typing hippo.health, you are accessing the value ("good") of the instance variable (health) of the object hippo.By appending () to your statement print hippo.health, you get print hippo.health(), which Python interprets to be: print the return value of calling hippo.health. But, the value of hippo.health ("good") is of string type ('str' object), which is not a function/method, and, therefore, it is not callable. Hence, a TypeError is raised.